Umm, you might want to find another shop. First it sounds like they did a poor job the first time. Did they not check for warped drums the first time? Would have shown up when they adjusted the brakes, if they even did. Did they not road test it?
Now it sounds like maybe they have the rear brakes adjusted too tight.
Machining rotors and drums can in some cases correct a warp, but remember you are removing material from them to do so, so now they have less material to absorb the heat.
And if you need brake drums, do it yourself. Lot less money, and plenty of how to it's on the web.
97 Sahara, 74 Renegade, 72 CJ5, 55 Willy's (the Beast) Boss 302, 6:34 gearing, Bose 901 speakers.
God I miss that Jeep. Forgot the kinda Jeep, 80 SX4.